Soldering is a common method for integrally bonding (joining) components. In this method, the components are bonded via a molten metal or metal alloy, also known as solder. In order to produce bonds that are as free of degassing and residues as possible, as is required, for example, in the area of micromechanical sensors, flux-free solders are used.
Typically pure nickel layers are formed on the parts to be joined (which often contain steel, nickel alloys, etc.), since such layers make obtaining stable intermetallic compounds possible. The problem, however, is that nickel layers have a strong tendency toward surface oxidation. Surface oxidation may occur even at room temperature and is promoted in particular by increased moisture or increased temperature. The formation of an oxide layer results in reduction of the flow characteristics and wetting characteristics compared to a (flux-free) solder, which interferes with the formation of a soldered bond. In extreme cases no bonding of the solder occurs at all. To prevent undesirable effects of this type, a nickel layer is normally provided with oxidation protection or tarnish protection. A gold layer is then normally formed on the nickel layer, so that a reduction of wettability by a solder is prevented.
The surface region on a component which is to be wetted by a solder is often defined or delimited. Therefore, the component is selectively coated with nickel and gold in order to limit wetting by solder to a defined surface region. For this purpose, a structured masking layer is formed on the component in question prior to forming the nickel layer, which results in the nickel and gold layers being formed only in regions between the masking layer. This procedure is, however, relatively complicated and therefore associated with high manufacturing costs.